1. Field of the Invention
The present invention relates to high voltage through type capacitors and manufacturing method thereof and, more particularly, relates to high voltage through type capacitors suitable for noise filters for magnetrons provided in electric cooking ranges.
Description of Related Art
Conventionally, through type capacitors for preventing radio noise from leaking from magnetrons used for electric cooking ranges are well known, for instance in Japanese utility model publications S55-35803/1980 (IPC:H01
Next, the construction of a conventional through type capacitor is explained with reference to FIGS. 33, 34(a) and 34(b).
The through type capacitor has a cylindrical dielectric 1 of through type made of a ceramic on both end faces of which first and second thick film electrodes 2 and 3 are formed by burning Ag paste at a temperature of 700 to 800.degree. C. A conductor 4 is arranged so as to pass through the cylindrical dielectric 1 and is electrically connected to the first electrode 2 by a connection plate 5 via solder. An enlarged ground metal plate 6 is electrically conducted connected to the second electrode 3 by a step portion 6b thereof forming an aperture 6a. The through conductor 4 is covered with an insulating tube 7.
These elements 1, 4, 5 are covered by a cylindrical outer case 9 made of a resin such as polybutylene terephthalate and insulating resin such as epoxy resin is filled into the space defined by the outer case 9 in order to protect those elements 1, 4 and 5. Further, a cylindrical insulating cover 10 covers the lower portion of the conductor 4.
Epoxy resin used as an insulation filler shrinks in a direction indicated by arrows upon curing, as shown in FIGS. 34(a) and 34(b) and, thereby, a small gap is formed at the boundary surface la between the cylindrical dielectric 1 and the epoxy resin filled inside of the dielectric 1. More particularly, the linear thermal expansion coefficients of the ceramic forming the cylindrical dielectric 4 and the epoxy resin are 1.about.10.times.10.sup.-6 /.degree. C. and 1.about.10.times.10.sup.-5 /.degree. C. respectively, and when a thermal shock test for the through type capacitors is performed, gaps and cracks are caused at the boundary 1a between the ceramic and epoxy resin by relatively strong residual thermal stress due to the big difference between the thermal coefficients mentioned above. This invites a concentration of charge and drop of the dielectric strength property. Further, in a humidity resistance test for the through type capacitors, humidity penetrates into the through type capacitors. In order to prevent the humidity from penetrating there into, the insulating tube 7 is provided.
The through type capacitor having the construction mentioned above is exposed to an a higher temperature environment since it is arranged in a shield case of the magnetron. In the electric cooking range, the magnetron is cooled, via cooling fins, by wind generated by a cooling fan and the through capacitor is also cooled by the wind. However, the anode portion of the magnetron is heated up to a high temperature of about 300.degree. C. and the capacitor is always exposed to a high temperature of 100 to 120.degree. C. and often to a higher temperature of about 150.degree. C. Further, accidentally, the capacitor may be heated up to such a high temperature as 180.degree. to 200.degree. C.
Moreover, since goods being operable as both the cooking range and oven are developed and the cooling fans having a low cooling faculty are used for saving cost, the heat resistibility of the through type capacitor is highly demanded.
However, epoxy resin is softened at a temperature of about 150.degree. C. and the degradation thereof progresses rapidly progressed to invites which invites peeling at the boundary surface. Thus, the conventional through type capacitors have a limit to the strict demand of the heat resistance mentioned above.
In order to solve the problem mentioned above, a resin exhibiting a heat resistance of about 200.degree. C. has recently been developed. However, the resin has a very high hardness and, therefore, cracks and breakages are caused due to residual stress generated upon shrinkage thereof and thermal shock and, also, gaps and peeling at the boundary surface are caused. These result in an inferior dielectric strength and poor reliability.
On the contrary, it has been known that silicone rubber has a high heat temperature resistance of about 200.degree. C. together with an elasticity. However, it has no adhesion and, therefore, it is impossible to use the silicone rubber as an insulating filler for the through type capacitor since humidity may penetrate thereinto through small gaps formed between the silicone rubber filled as the insulating filler and the ceramic dielectric 1 resulting in poor insulation.
In order to solve the problem mentioned above, a method for applying a primer to respective elements to be covered by the filler is considered. However, it is impossible to obtain stable dielectric and humidity resistances since an extra step for applying the primer is needed and control of the primer layer is difficult.
Also, the dielectric resistance of an order of ten and several KV is required in the through type capacitor since a high voltage of 7 to 8 KV is applied thereto upon driving the magnetron.
As is apparent from the mentioned above, high dielectric resistance, heat resistance and resistance to thermal shock are required for the through type capacitor.
Further, the through type capacitor is required to have a high tracking resistance as one of the important properties to be owned thereby. This tracking resistance is used to evaluate the dielectric strength in such a case that water is condensed on the surface of the through type capacitor by a rapid and big change in the temperature. In this regard, the insulating case and cover 9 and 10 made of an organic high-polymer conventionally used have poor tracking resistance since the high-polymer is carbonized once a high voltage is leaked along the outer periphery of the capacitor and, thereby, the capacitor comes into a short circuited state.
As mentioned above, since silicone rubber is used as the insulating tube 7 and, therefore, it is high in cost and is laborious because of work for inserting the through conductor thereinto, the cost of the capacitor becomes high. Also, the insulating property of the capacitor using epoxy resin as the insulating filler is worsened rapidly at a temperature higher than 150.degree. C. which is an upper limit of use with respect to the epoxy resin.